A Study of Modern and Geologic Erosion Rates in the Colombian Andes by ...

Fourth LACCEI International Latin American and Caribbean Conference for Engineering and Technology (LACCET'2006) "Breaking Frontiers and Barriers in Engineering: Education, Research and Practice" 21-23 June 2006, Mayag?ez, Puerto Rico.

A Study of Modern and Geologic Erosion Rates in the Colombian Andes by Low-Temperature Thermochronology, 10Be Analysis, and GIS/RS-based

Applied Geomorphology: An Attempt to Adhere to "a New Social Contract for the Sciences"

Sergio A. Restrepo M. Ph.D. Candidate Department of Geological Sciences, University of Florida, Gainesville, Florida, USA, sergiorm@ufl.edu

Abstract

Whether occurring at its "natural" pace or enhanced by human activities, erosion is an important physical phenomenon. Form a "purely scientific" point of view, erosion deserves to be better understood because it controls many other physical processes across a wide range of spatiotemporal scales. From a more "practical" point of view, erosion deserves even more attention because myriad facets of current environmental degradation are influenced by erosion. Due to human activities, we are experiencing a time of exacerbated erosion rates in Earth's history. A novel approach (involving low-temperature thermochronology, 10Be analysis, and GIS/RS terrain analysis) is used in an attempt to quantify and compare pre- and post-disturbance erosion in the Colombian Andes. This study is also an attempt to adhere to "a new social contract for the sciences". However, the staggering figures about anthropogenic erosion rates (1000X > than geologic rates) derived from sophisticated research techniques cannot solve the actual problem of enhanced soil erosion and depletion of other natural goods and services. Scientific-based information is not enough. Complex social dynamics determine what soils are lost, and even a new social contract for the sciences seems unable to solve the problem. What path should the scientific/academic community take?

Key Words Anthropogenic-erosion, thermochronology, 10Be, science, society

1. Introduction Earth's surface processes are those that take place at the interface of the spheres (i.e., geosphere, hydrosphere, atmosphere, and biosphere). Although neglected for some time in geological sciences, it is now recognized that such processes have played a crucial role in the evolution of the Earth, are complex both spatially and temporally, and are characterized by feedbacks and fluxes of matter and energy between the spheres (Phillips, 1999). Erosion by water is one such process and it affects the evolution of landforms and soils (Owens and Slaymaker, 2004), the rate of regolith removal and transfer of sediment to water bodies (Meybeck et al., 2003), the morphotectonic evolution of orogenic belts (Burbank and Anderson, 2001), the composition of the atmosphere and ocean, (Raymo and Ruddiman, 1988), and changes in climate trends (Molnar and England, 1990), etc. Changes in climate, topography, vegetation cover, base level, etc. also influence erosion. In recent years erosion has become a fundamental line of scientific enquiry capturing the attention of scientist working in numerous disciplines such as geomorphology, environmental geology, tectonics, pedology, oceanography, and climatology.

But erosion is more than just an interesting scientific matter. In fact, due to human modification of the natural environment (e.g., deforestation), enhanced-erosion is one of the most pressing, worldwide environmental problems as it affects food security, hydrologic resources, and infrastructure (Goudie, 1995). The majority of human-induced changes in the rate and nature of geomorphic processes are detrimental to the sustainability of the environments that support society. In the tropics, where the

majority of ecological "hotspots" are located (Mittermier et al., 2000), the combination of natural factors (e.g., high rainfall, steep terrain, active tectonics, etc.) and human-mediated processes (e.g., deforestation, development of infra-structure, etc.) result in some of the highest erosion rates in the world. Accelerated erosion usually triggers severe environmental degradation.

Relative to the geologic time frame, disruption of natural geomorphic dynamics and the consequent increase in erosion rates are fairly recent phenomena (Page and Trustrum, 2000). The Holocene brought with it the emergence of a new sphere, the technosphere; the realm of the human activities (e.g., agriculture, housing, industry, mining, energy, etc.). Processes within the technosphere have significantly modified the interactions between the spheres. Enhanced erosion is a direct consequence of this late Holocene scenario, and just as we thrive to understand past climate in order to comprehend the degree to which humans have perturbed the climate system, we must also cautiously study past erosion in order to understand the degree of perturbation of natural geomorphic dynamics. And, as is the case for climate, erosion is also a domain of inherently high complexity. Therefore, addressing erosion rates at several spatiotemporal scales requires the use of sophisticated tools ranging from those needed to remotely acquire and analyze terrain data (e.g., DEM, multispectral imagery, etc.) to those utilized in isotopic analysis (e.g., mass spectrometers, scanning electron microscopy, microprobes, etc.). The theoretical developments and research methods used today in quantitative geomorphology are equally sophisticated.

1.1. Controls on Erosion Rates

Of the variables that control erosion rates (e.g., topography, vegetation, substrate erodibility, and climate/rainfall erosivity) vegetation cover is the one most susceptible to influence by humans (Owens and Slymaker, 2004; Restrepo, 2003, Thornes, 1994; Vis, 1991). We live in a planet heavily impacted by human activities. Massive deforestation has occurred over the last few centuries in the tropics. This activity continues today at a rate of ~30 ha*minute-1, a clear indication that our species is a prominent force within the Earth system.

It has been suggested that humans are now the greatest geomorphic agent (Owens and Slaymaker, 2004; Hook, 2000). In the Tropical Andes of Colombia (TAC), an important ecological "hotspot" (categorized by Mittermeier et al., (2000) as the global epicenter of biodiversity), the combination of natural factors and human-mediated processes cause some of the highest erosion rates in the world (Thomas, 1994; ElSwafy et al., 1982). In the TAC, ~95% of the natural vegetation has been destroyed. Inappropriate agricultural practices render soils even more vulnerable to erosion. Disrupted ecosystems exhibit altered hydrological and erosional dynamics and their long-term integrity is threatened (Mittermeier et al. 2001).

2. Geologic vs. Anthropogenic Erosion: Science with Immediate Social Applications

It has been hypothesized (Restrepo 2003) that average modern erosion rates in the TAC are three to four orders of magnitude higher than long-term, pre-disturbance erosion rates; an increase mainly attributable to anthropogenic activities. To test this hypothesis, a multidisciplinary approach to quantify geologic and recent erosion in the TAC is being implemented.

Despite the general acceptance of the anthropogenic origin of accelerated erosion, it remains difficult to tackle long-term erosion rates based solely on contemporary data. This is mainly due to the pervasive human impact on vegetation cover and slope hydrology. Moreover, estimates of erosion derived from fluvial sediments are spurious as sediment load fluctuates dramatically with time due to variations in discharge and sediment availability (Douglas, 1967). This has led to a dominance of qualitative approaches in studies of long-term erosion dynamics, which precludes reliable estimates of "baseline" erosion rates against which the effects of anthropogenic perturbation can be compared. The research

project presented here involves the use of low-temperature thermo-chronology (LTTC), terrestrial cosmogenic nuclide analysis (TCNA), and geographic information systems/remote sensing-based applied geomorphology (GIS/RS-AG) to quantify erosion rates at three different time scales: long- (107-106 years), mid- (106-103 years), and short-term (102-101 years), respectively. This approach permits to overcome the difficulties mentioned above and allows a quantitative evaluation of anthropogenic impact on erosional processes against the estimated background rates of substrate stripping. In addition, results from this study will also shed light on the long-term patterns of landscape evolution and regional exhumation/uplift of an elevated plateau situated in an active mountain chain that is regionally important.

The way in which this investigation has been designed makes it not only scientifically challenging, but also allows making fundamental social contributions. A quantitative comparison of geologic and anthropogenic erosion rates will help raise environmental awareness by highlighting the critical role humans play in accelerating erosion rates and triggering environmental degradation manifested as loss of fertile soil, pollution of water bodies, destruction of terrestrial and aquatic ecosystems, deterioration of infrastructure, and alteration of climate/weather patterns.

3. A Research Strategy that Attempts to Fit Within "a New Social Contract for Science"

"It is time to take stock of the concepts, data and methodologies that can be applied now or in the very short-term to address several troublesome questions". Stone, 1979.

Traditionally, geologists have considered geomorphology more as a branch of physical geography than a geological discipline per se. However, three circumstances have lead to an increased mixture of geology and geomorphology. First, the rising acceptance of the intricate coupling between surface processes (geomorphology) and the morphotectonic evolution of geologic provinces, which resulted in the emergence of "tectonic geomorphology", one of the most interdisciplinary scientific disciplines (Burbank and Anderson, 2001). Moreover, the quantitative study of landscape evolution (at various scales of space and time) has blossomed over the last decade, mostly driven by the prevalent interdisciplinary interest in discovering the nature of potential interactions (at various scales of space and time) between climate, surface processes, tectonics, and, more recently, anthropogenic activities. Second, the recognition that all environmental and human systems are interlinked and that studying complexly interlinked systems in isolation is dysfunctional. And third, the realization that humans are a geologic force, and that, over a rather short time span, anthropogenic activities (i.e., the technosphere) are triggering uncontrollable and unpredictable readjustments of the other spheres (e.g., mass extinction, deforestation, ozone depletion, global warming, sea level rise/glacial melting, etc.).

Academic programs on geomorphology at the graduate level are now offered by geology and geography departments alike, providing unparalleled opportunities to establishing multidisciplinary research and to approximate sophisticated geological and geographic technologies (isotope geology, GIS/remote sensing, etc.) useful in studying both geologic and environmental matters. Courses of environmental geology, now spread across campuses, have resulted in large part from the hybridization of geology and geography. A further positive outcome is the emergence of a more interdisciplinary academic community that, through a more open dialogue, strives to comprehend the complexities of Earth's systems in order to plan a more judicious manner to operate the technosphere. In this investigation, the bridging nature of geomorphology and the proximity between geology and geography are exploited to address relevant geologic (landscape evolution, active orogenic belt exhumation, plateau development, etc.) and environmental problems (anthropogenically enhanced erosion rates, land use effects on geomorphic processes, etc.).

The evident nature of the adverse environmental effects of erosion on soils, biota (terrestrial and aquatic), and water resources make it a crucial environmental issue. Richter and Markewitz (2001) made the point that "humans are increasingly living in urban and suburban environments, away from the land and apart from the soil, yet the quality of human life and the Earth's environment has never depended more on soil management than it does today. Soil deserves a much greater share of human attention and affection...

Soil is the central processing unit of the Earth's environment". The costs associated with worldwide soil erosion are estimated to be ~$500 billion*y-1 as a result of direct damage to agricultural lands and `indirect' damage to waterways, infrastructure, and health (Pimentel et al., 1995). Undoubtedly these values would increase if other deleterious effects on the environment were considered (e.g., destruction of fisheries, desert expansion, etc. Restrepo, personal communication).

Within the framework just presented for this investigation it seems feasible to satisfy one's passion for science while simultaneously providing something useful to society. In other words, it becomes relatively simple to adhere to a new social contract for science as postulated by Lubchenco (1998): "Urgent and unprecedented environmental and social changes challenge scientists to define a new social contract. This contract represents a commitment on the part of all scientists to devote their energies and talents to the most pressing problems of the day, in proportion to their importance, in exchange for public funding".

3.1. The Research Strategy: A blend of Geology, Geomorphology, and Geography.

Although there is general agreement about the great intensity of modern erosion in the TAC, quantitative data on pre-disturbance rates are still scant. Understanding the degree to which humans have exacerbated erosion rates is crucial to raise the awareness in relation to soil and water depletion, a pressing environmental problem. However, a thorough understanding of erosion rates has often been limited by a lack of quantitative data, making it virtually impossible to understand the role of humans as a geomorphic agent or to discern patterns of landscape evolution.

Reconstructing long-term erosion histories by extrapolating current trends in erosion derived either from river sediment loads or experimental plots is spurious due to both the extent of human perturbation of natural geomorphic systems and the stochastic nature of sediment removal and transport (Trimbley, 1977). Measurements of erosion rates derived from AG studies (e.g., erosion pins, sediment load, sediment traps, experimental plots, etc.) carried out over short temporal scales (e.g., 100-101 years) are strongly influenced by anthropogenic perturbation of natural geomorphic dynamics (Douglas, 1967).

To the contrary, quantitative data for erosion rates generated for geologic temporal scales (i.e., 103 ? 108 years) by using LTTC and TCNA provide reliable information about erosion rates free of anthropogenic signals (Burbank and Anderson, 2001; Gosse and Phillips, 2001; Farley, 2002). Such an approach permits defining a base-line of erosion so that pre-Holocene (i.e., geologic) and modern (i.e., anthropogenic) erosion rates can be quantified and compared, which allows us to increase our understanding about the impact of humans on natural geomorphic systems. For this investigation, erosion rates are quantified for three different time scales: long- (LT), mid- (MT) and short-term (ST), that is to say, from tens of millions to tens of years. The geologic (i.e., `natural') rate of erosion will be measured and compared to modern (i.e., anthropogenically-enhanced) erosion rates.

Physiographic characteristics of the Altiplano Antioque?o (elevated plateau incised by the Medell?n/Porce fluvial system, granodioritic batholith, anthropogenic perturbation of natural ecosystems via deforestation, etc.) offer an excellent scenario to reconstruct and compare geologic and modern erosion rates by using LTTC, TCNA, and GIS/RS-AG. This part of the study demonstrates the usefulness of helium dating to quantify long-term erosion rates and exhumation of the Altiplano Antioque?o. An integrated application of LTTC, TCNA and GIS/RS-AG can not only revolutionize our understanding of erosion, but also provide a quantitative basis to support our models of morphotectonic evolution in diverse geologic provinces. Because TCNA and GIS/RS-AG are still in a very early stage of implementation for this research, only preliminary results of apatite helium dating are presented.

3.1.1. Low Temperature Thermochronology LT data (106-107 years) are derived from high-resolution LTTC, specifically the systems (U-Th)/He (partial retention zone-PRZ ~ 40-80?C), and fission tracks in apatite (partial annealing zone-PAZ ~110-

60?C). Helium analysis is based on measurements 4He (emitted and retained particles) resulting from the radioactive decay of 235U, 238U y 232Th. Thermokinetics of helium diffusion and -particle stopping distances in apatite (~20?m) are well constrained. Thus, measurements of U, Th and He by mass spectrometry allow calculating the closing time of the system. The equation that defines the accumulation of 4He through time is (Farley, 2002):

4He = 8*[238U(exp(238 t) -1)] + [7/137.88]*[238U(exp(235t) -1)] + 6*[232Th(exp(232t) -1)]

Where 4He, 238U, and 232Th represent the amounts of such isotopes as measured in the sample by mass spectrometry, t is the accumulation time (retention) or radiometric age, and s are the radioactive decay constants for each isotope.

Apatite fission-track analysis relies on the measurable amount and length of lattice defects (tracks) induced by spontaneous nuclear fission of 238U (Gallagher et al., 1998). These two thermochronometers possess the lowest closure temperatures known. LTTC-derived apparent ages obtained from samples collected along vertical profiles (e.g., canyon walls/scarps) and analyzed against elevation provide a measure of long-term erosion rates because both systems record the time at which a rock cools through the last 3 km of the crust (~ 60-90?C), a domain strongly influenced by surficial processes such as erosion (Burbank and Anderson, 2001).

A Preliminary examination of the data derived from helium analysis for Matasanos and La Garcia profiles shows a very well behaved data set with excellent reproducibility of helium ages and very little dispersion in the apparent age vs. elevation plots. Helium ages decrease systematically with profile depth for all of the samples collected along the Medell?n-Porce scarps from 48.9 ? 2.4 Ma to 22.84 ? 1.14 Ma. Such behavior is in agreement with theoretical profiles and apparent age vs. elevation diagrams. Exhumation curves clearly display the inflection points, and the characteristic shape of the exhumed PRZ-helium. The first segment of the cooling curves for both profiles indicates a typical rapid-exhumation period starting at about 25 Ma. The similarity in the age versus elevation distribution is a sign of very good internal consistency of both data sets and can be interpreted as the exhumation of the entire plateau as a continuous, discrete unit. Even though the samples analyzed represent the full extent of the crustal section exposed along the northern scarps of the Medell?n/Porce fluvial system, fission-track data is required in order to reconstruct the paleo-geothermal gradient, which will permit a more accurate estimation of erosion rates. The period of quiescence represented by the "shallow" segment of the profiles gives an average erosion rate of about ~0.037 mm*y-1 that prevailed from ca. 45 to ca. 25 Ma (~ 20 Ma). Such rates are well below estimated modern rates ~80 mm*y-1.

3.1.2. Terrestrial Cosmogenic Nuclide Analysis (10Be) Quantification of MT erosion rates (106-103 years) will be based on TCNA of 10Be. Terrestrial cosmogenic nuclides (e.g., 10Be, 26Al) are produced by the interaction of cosmic rays (mainly neutrons) with a variety of target atoms, e.g., Si and O in quartz. Neutrons are rapidly attenuated with depth in the uppermost crust, and hence, can be employed to monitor near surface processes (e.g., erosion, geomorphic exposure). Nuclide activities of a sample, as measured by accelerator mass spectrometry, are greater for materials collected on geomorphic surfaces with low erosion rates/long exposure times (Goose and Phillips 2002). For this investigation, ten site-specific samples of rock, soil and regolith obtained on erosional surfaces will yield erosion rates at discrete points on the landscape. In addition, twelve samples of active sediment from small fluvial systems will be used to determine spatially averaged erosion data at the basin scale.

3.1.3. GIS/RS-Based Applied Geomorphology Reconstruction of ST erosion rates (106-103 years) is in progress. A GIS/RS-AG model of spatial patterns of erosion that incorporates the variables that exert the greatest control on erosion is being constructed. The potential of GIS/RS data and digital terrain analysis in geomorphologic studies has been stressed

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